Coupling light from one optical fibre to another can be achieved by placing two parallel fibres in close proximity so that the mode fields of the fibres overlap. This is the principle of the known parallel fibre coupler in which the two fibres are identical and light periodically couples out of the launch fibre and into the other fibre. If the material of the fibre is non-linear in that its refractive index varies with light intensity, then the coupling characteristic is altered. A non-linear parallel fibre coupler has been proposed, consisting of two identical fibres of non-linear material, the predicted coupling characteristics of which are similar to those of the linear coupler up to a critical power, above which the degree of coupling is much reduced. At the critical power a steady, equal power distribution is obtained. This critical power phenomenon enables the coupler to exhibit two distinct output states for small changes in input power so that the device may be used, for example, as an optical switch or amplifier.
Non-linear parallel fibre couplers made from currently available materials would require comparatively long interaction lengths in order that the critical power can be reached. Hence, the elements would need to be of the order of 1 cm and it presents a significant technological problem to fabricate accurately parallel fibre guides for that length. Also at the critical power the power division is even which is not optimal for all purposes.
Accordingly the present invention provides an optical coupler comprising at least two waveguides positioned in coupling proximity, wherein the waveguides comprise nonlinear refractive index material and support non-identical mode fields and wherein the waveguides are such that the value of the overlap integral of the nonlinear reaction of each waveguide with itself is in a predetermined ratio to that overlap integral of the other waveguide to provide predetermined critical power characteristics of the coupler.
Another aspect of the invention provides an optical coupler comprising at least two waveguides positioned in coupling proximity, wherein the waveguides comprise nonlinear refractive index material and support non-identical mode fields and wherein the waveguides are such that the values of the overlap integrals of the nonlinear reaction of each waveguide with itself have a maximum normalised difference value of +0.12.
Preferably, to keep the required power levels within reasonable limits the minimum value should be of the order of -0.5. Values in the range between -0.12 and +0.12 are considered desirable.
Within the context of this specification nonlinear refractive index refers to the non-linearity that occurs due to the Kerr effect in which the refractive index varies with field strength.
In a preferred embodiment the waveguides are coaxially disposed.
Power may be launched into either waveguide with some embodiments, or alternatively the asymmetric nature of the critical phenomenon in embodiments with a difference in said overlap integrals may be exploited by launching power into a particular one of the waveguides.